The present invention relates to a fuel injection device for an internal combustion engine.
A construction of a fuel injection system in the prior art is illustrated in FIG. 1. In this figure, reference numeral 10 designates a fuel injection pump main body, numeral 20 designates a plunger, numeral 30 designates a delivery valve, numeral 31 designates a delivery valve spring, numeral 32 designates a delivery valve chamber, numeral 40 designates a fuel injection pipe, numeral 50 designates a fuel injection valve main body and numeral 51 designates a nozzle tip portion.
Now description will be made of the operation of the above-described system in the prior art. The plunger 20 is driven by a cam (not shown), then compressed fuel raises the delivery valve 30 against the spring 31 and enters the delivery valve chamber 32, further it generates a pressure wave within the injection pipe 40, this pressure wave enters the fuel injection valve 50 to push up an automatic valve (not shown) provided within the valve, and the fuel is injected into an engine combustion chamber through the nozzle injection hole at the nozzle tip end portion 51.
The main construction of this fuel injection system is diagrammatically shown in FIG. 2, and the injection system in the prior art generally has a fuel injection pipe whose cross-sectional area (or inner diameter) is uniform over the entire length.
The injection system in the prior art has an injection hole choke at the tip end of the fuel injection pipe having a uniform cross-sectional area. As a result, the pressure wave propagated through the fuel injection pipe rises in pressure at the injection hole section and thus provides injection. However, at that time, a part of the energy of the pressure wave is reflected and returns to the side of the fuel injection pump where it is again reflected, resulting in secondary injection. Representing the cross-sectional area of the injection pipe by A.sub.p and the cross-sectional area of the nozzle by A.sub.N in FIG. 2, the magnitude of the reflection wave becomes large as the ratio A.sub.p /A.sub.N is increased, and secondary injection is liable to occur. In order to prevent this phenomenon, a large amount of suction back function is necessitated at the delivery valve section on the pump side, but if the amount of suction back is too large, cavitation would be generated. For the purpose of preventing this shortcoming, if the ratio A.sub.p /A.sub.N is chosen small and A.sub.p is reduced, then generally cut-off at the end of injection can be improved, but the pressure on the pump side rises and hence a reduction is liable to occur in the durability of the cam and the pump. It is to be noted that in FIG. 2 reference character A.sub.PL represents a cross-sectional area of a plunger, reference character L.sub.p represents a length of the injection pipe and reference character P.sub.o represents an open valve pressure of the nozzle.
As described above, in the fuel injection system in the prior art having a fuel injection pipe with a uniform cross-sectional area, secondary injection or cavitation is liable to occur, and in the case of a thin fuel injection pipe, the pressure loss is increased and hence the pressure on the pump side is increased, while in the case of a thick fuel injection pipe, the pressure falls slowly and hence cut-off of the injection is not good.